A shape grammar is a collection of visually defined geometric rules that could be used to automate the generation of formal representations of designs for buildings, cities, products and more. We offer an extension of the shape grammar formalism based entirely on voxel space instead of vectors, which we used for the generation of schematic architectural designs. We describe a method using playability to increase human agency and designer control over the outcome of the generative phase of voxel-shape grammars. The method is presented with an implementation in the environment of Minecraft and employs three guidance mechanisms. To conclude we list a few considerations from our experience in the design of a playable, voxel-shape grammar and point to future work.